(1) Field of the Invention
The present invention relates to a coder and a coding method for coding multivalued image data in which the pixel value of one pixel is expressed by a plurality of bits, a program for realizing the coding processing of the multivalued image data, and an image forming apparatus.
(2) Description of the Conventional Art
Recently, an image forming apparatus equipped with a so-called memory recall function has been realized. The image forming apparatus with the memory recall function stores in a memory the image data of an image for which image forming has been performed and reads the image data from the memory for image forming when receiving user instruction to perform image forming again, for instance.
With the image recall function, an original document needs not to be read again using the image reader and the like or the image data needs not to be transmitted again from an external apparatus such as the personal computer (referred to as the “PC” in this specification) when the image data stored in the memory is used. Accordingly, the larger amount of image in the memory, the more improved usability.
However, a larger memory capacity for a larger amount of stored image raises the manufacturing cost of the image forming apparatus. This is problematic. This problem may arise when the number of pages to be sorted is increased for the so-called electronic sorting function by which image data is once stored in the memory and a plurality of printed papers are sorted when output. Also, the problem noticeably arises for the full-color image forming apparatus that stores image data in the memory for each of the reproduction colors, yellow, cyan, magenta, and black.
In order to prevent the manufacturing cost raise due to memory capacity increase, image data is compressed when stored in the memory. For this purpose, a variety of methods of coding image data have been developed. Here, an explanation of coding processing will be given as an example of coding method as a related art. According to the coding method, a plurality of bit planes are generated from multivalued image data in which the pixel value of one pixel is expressed by a plurality of bits. Also, each of the generated bit planes is coded.
FIGS. 1A and 1B are diagrams for explaining the above-mentioned related art. According to this method, multivalued image data 900 with 256 gradation levels in which the pixel value of one pixel is expressed by 8 bits as shown in FIG. 1A (only 16 pixels are illustrated in FIG. 1A) is divided into 8 bit planes 901 to 908 as shown in FIG. 1B. Then, the multivalued image data 900 is coded by the bit plane using the method of arithmetic coding stipulated by the JBIG (Joint Bi-level Image experts Group), for instance.
However, coding by the bit plane is also problematic. For instance, among the 8 bit planes 901 to 908, the pixel value distribution in the original image almost remains in the bit plane 901 that consists of the most significant bits. As a result, relatively a good compression ratio is realized even in the arithmetic coding. On the other hand, in the order of the bit planes 902, 903, 904, . . . , and 908, the bits in a bit plane become less significant, and a bit plane has less relation with the original image. It is well known that the bit plane consists of less significant bits shows a so-called “white noise pattern”. The white noise pattern is a random image totally different from the natural image in characteristics and properties. As a result, there is a limitation in improving the compression ratio according to the arithmetic coding, for instance.